This invention relates to a method and apparatus for drawing fibers, yarns, or tapes formed from natural resin, synthetic resin, or combination of both. For the sake of convenience, in the description of this invention, which follows, the method and apparatus will be described in terms of drawing fibers. However, it is to be understood that the method and apparatus are equally usable for drawing any elongated body elements subject to such procedure. But for all that, the invented drawing method and apparatus will be described in terms of drawing of fibers in the form of coiled fiber loops. It is to be understood that the coiled fiber loops are a connected set of rings or twists into which the fiber can be wound.
In the production of most polymer fibers (e.g., nylon, polypropylene, and polyester fibers) a drawing stage is included subsequent to the spinning or extrusion stage. In the drawing stage the fiber is usually drawn by a drawing apparatus at elevated temperature to a length substantially exceeding (in some cases, several times) their original length. The fiber passes the drawing apparatus with a speed Vfiber which increases from the beginning to the end of the drawing stage, speed Vfiber being a linear speed along the fiber axis of fiber points in the drawing process (along the tangent to the fiber axis if the fiber drawing occurs on the curved surface, e.g., a curved hot plate or a roller). A fiber draw ratio λ, which is the extent of fiber drawing, is given byλ=Vfiber2/Vfiber1,  (1)where                Vfiber1 is fiber speed Vfiber in the beginning of the fiber drawing stage, and        Vfiber2 is fiber speed Vfiber at the end of the fiber drawing stage.        
The drawing of the fibers enables them to achieve the required molecular orientation and structure by virtue of which they attain the necessary strength and other desired physical characteristics. As an example, typical λ for commercial nylon fibers is about 6 to 1. Usually, the higher λ, the higher molecular orientation and fiber tensile properties (tenacity and Young modulus in particular).
In case of a continuous multi-stage drawing process, Vfiber2 is the fiber linear speed at the end of the last fiber drawing stage, and Vfiber1 is the fiber linear speed in the beginning of the first fiber drawing stage.
The drawing has generally been hitherto effected on a commercial scale by passing the fiber from one set of rotating rollers to another. Each set of receiving rollers rotates at a surface speed, which is greater than that of the preceding set of feed rollers.
In case of drawing the fiber by the rotating rollers we getVfiber1=Vsurface1=Vinlet and  (2)Vfiber2=Vsurface2=Voutlet,  (3)where                Vsurface1 is a linear surface speed of the feed rollers,        Vsurface2 is a linear surface speed of the receiving rollers,        Vinlet is a fiber inlet speed, which is a fiber linear speed along the fiber axis of feeding the fiber to the feed rollers, and        Voutet is a fiber outlet speed, which is a fiber linear speed along the fiber axis of conveying the drawn fiber from the drawing stage either to a next stage of the continuous fiber making process (drawing, heat setting, relaxation, bulking or texturing, twisting, finish application, etc.) or to a receiving package.        
Thus in conventional industrial drawing processes a ratio of fiber outlet speed Voutlet to fiber speed Vfiber2 is 1 to 1 (this ratio will be used for discussion below).
In case of conveying drawn fiber after the drawing stage to the receiving package we getVfiber2=Voutlet=Vtake-up,  (4)where                Vtake-up is a take-up speed, which is a fiber linear speed along the fiber axis of taking up the drawn fiber on the receiving package (in some cases, Vtake-up is slightly higher than Vfiber2 and Voutlet to provide some tension in the taken-up fiber).        
Outlet speed Voutlet and take-up speed Vtake-up determine the throughput of the drawing stage. Most conventional commercial processes, particularly in the area of melt-spun flexible-chain polymer fibers, have very high speed Vtake-up ranging from several hundred to several thousand meters per minute to provide high throughput. This means that in the high-throughput commercial processes speeds Vfiber2, and Voutlet are also high, i.e., ranging from several hundred to several thousand meters per minute.
Another parameter is used to characterize the fiber drawing process, i.e., a speed of drawing or a strain rate Vstrain, which is a relative deformation of the fiber (strain) in a unit time. Usually strain rate Vstrain is expressed in percent per second (%/sec) and is given byVstrain=λ/T,  (5)where                T is time of drawing.        
In conventional commercial processes strain rate Vstrain is high, i.e., several hundred percent per second.
In such conventional high-fiber-speed, high-drawing-speed processes the fiber is subjected to a very abrupt acceleration and rise in tension at the point where it leaves one roller to pass to the succeeding higher-speed roller. Care must be taken to ensure that the abrupt rise in tension does not break the fiber. Thus, this conventional technique may be termed “impulsive drawing” because the fiber experiences a sudden “impulsive” acceleration from its initial state to its final drawn state while traveling through the drawing machine. The “impulsive” acceleration and high tension result in frequent fiber breaks and equipment stops, high volume of waste, and preventing further fiber improvement.
Because of high fiber speed, time of drawing T is very short in most high-throughput industrial processes, i.e., less than a second for one-stage drawing and about 1-3 seconds for two- or three-stage drawing. This results in “non-equilibrium” drawing where the fiber does not have enough time to be heated to ambient elevated temperature while being drawn, and the drawing occurs at high temperature gradient in the fiber cross-section. This, in turn, results in reduced drawability and crystallinity, high gradient of morphology and physical properties in the cross-section, high local overstresses, reduced tensile properties, and dimensionally unstable fibers with high hot-air shrinkage. This is especially typical for fibers and yarns having high denier (denier is weight in grams of 9000 meters of fiber). To provide additional time for heat setting, the existing technology requires a separate, specialized, very expensive, and energy-consuming equipment to produce dimensionally stable fibers without decrease of their tensile properties (U.S. Pat. No. 5,522,161 to Vetter (1996), U.S. Pat. No. 5,588,604 to Vetter et al. (1996)—these patents are discussed below). More often, a different method for decreasing shrinkage is used in commercial processes. The fiber is subjected to restricted shrinkage while moving through a special stage, which follows the last drawing stage. In doing so, the initial modulus, intermediate moduli, and tenacity are reduced.
The commercial drawing processes mentioned above do not enable one to produce polymer fibers with tensile and other physical properties close to those made by lab-scale low-fiber-speed, low-drawing-speed, long-drawing-time, and non-impulsive drawing process. This lab-scale drawing may be termed “uniform” or “equilibrium” drawing, where drawing time T is long enough to heat the fiber to the ambient temperature, while it being drawn, with low temperature gradient in the fiber cross-section. This results in uniform morphology and physical properties in the cross-section. These lab-scale experiments achieve more effective morphological transition “low-oriented-high-oriented polymer system” and superior physical properties. For example, tenacity of lab-scale flexible-chain, regular-molecular-weight, melt-spun polymer samples is higher by a factor of about 1.5-2 and initial moduli are several times higher than those for conventional commercial fibers. (As an example, tensile properties of lab-scale polypropylene fibers can be seen in “Superdrawn Filaments of Polypropylene” by W. N. Taylor, J R. and E. S. Clark, Polym. Eng. Sci., 18, 518-526 (1978). A comparison of these results with tensile properties of commercial polypropylene fibers is presented in Table VI below). In order to overcome this large gap between the properties of lab-scale and commercial-scale polymer fibers, a new approach needs to be developed.
Moreover, within today's fiber industry there is another large gap, i.e., tensile properties of low-cost, low-performance, regular-molecular-weight, melt-spun, flexible-chain polymer fibers (e.g., polyethylene, polypropylene, polyester, nylon, etc.) are much lower than those of high-cost, high-performance, wholly-aromatic polymer fibers (e.g., Kevlar® 49, DuPont and Twaron®, Teijin) and ultra-high-molecular-weight, solution-spun, aliphatic polymer fibers (e.g., Spectra®, Honeywell and Dyneema®, DSM). The great challenge for fiber science and technology is to fill this gap by producing industrially a new generation of low-cost, high-performance polymer fibers (most probably, flexible-chain, regular-molecular-weight, melt-spun) with substantially improved tensile and other physical properties. This can be done by introducing the results of the lab-scale research efforts (mentioned above) to the industry. It would be extremely attractive to achieve in the high-throughput industrial process (i.e., with take-up speed Vtake-up ranging from several hundred to several thousand meters per minute) fiber tenacity of about 1-2 GPa (12-22 gpd) and initial tensile modulus of about 20-100 GPa (250-1000 gpd) for different flexible-chain, regular-molecular-weight polymer fibers having different theoretical values of tensile properties.
In the work of Taylor and Clark mentioned above, tenacity about 1 GPa (12 gpd) and initial modulus 22 GPa (270 gpd) where achieved for melt-spun, regular-molecular-weight polypropylene filaments in the lab-scale experiments (see Table VI below).
Any company that makes progress in this area will have a tremendous advantage in competition today and in the future. To the best of our knowledge, no significant progress in this area has been so far achieved by the American, Asian, or European fiber industries.
A few attempts have been made in the prior art to improve conventional industrial drawing methods.
A method and apparatus for incremental drawing of fibers on the industrial scale were introduced in U.S. Pat. No. 2,372,627 to Goggin et al. (1945), in U.S. Pat. No. 2,788,542 to Swalm et al. (1957) and in U.S. Pat. Nos. 3,978,192 (1976), U.S. Pat. No. 4,891,872 (1990), U.S. Pat. No. 4,980,957 (1991), U.S. Pat. No. 5,339,503 (1994), and U.S. Pat. No. 5,340,523 (1994), all to Sussman. The incremental drawing improves the conventional commercial drawing process by dividing it into small steps, typically 10-30, i.e., fibers are drawn on microterraced or smooth surfaces of a pair of conical rollers with canted axes.
U.S. Pat. No. 4,967,457 to Beck et al. (1990) disclosed an arrangement for stretching thermoplastic fibers. In this patent fiber moves through a plurality of non-driven rollers arranged between the delivery mechanism and the stretching mechanism inside the heat chamber. Some rollers have brakes providing several successive stretching zones.
Both invented methods provide longer drawing path and drawing time T as well as lower strain rate Vstrain in comparison with conventional methods. However, as in the conventional drawing methods the fiber, while being drawn, passes the drawing apparatuses at high speed and at the end of the drawing stage Vfiber2=Voutlet (in other words, the ratio of outlet speed Voutlet to fiber speed Vfiber2 is 1 to 1). If after the drawing stage the drawn fiber is conveyed to the receiving package, Vfiber2=Voutlet=Vtake-up. Economical reasons force to keep speeds Vfiber2, Voutlet, and Vtake-up as high as possible, i.e., in the range from several hundred to several thousand m/min, in order to provide high throughput.
For both methods the “impulsive” acceleration, although reduced, remains high at each drawing step or zone. In case of high-throughput processes, the drawing is “non-equilibrium”, i.e., it still has short drawing time (a few seconds), which is not enough to heat the fiber (especially high-denier fiber) to the ambient temperature in the process of drawing. In case of the incremental drawing, the fiber is drawn only between rollers and not on their surfaces while traveling through the drawing apparatus. This results in reduction of drawing time to the level, which can reach about half of the residence time in the apparatus. The drawing starts and stops while the fiber moves through the drawing apparatus. Thus, the incremental drawing is not uniform and may be termed “intermittent drawing”.
A technology for winding fiber into coiled loops around a conveyer device, conveying these fiber loops at a slow speed and high residence time through a heat chamber by this conveyer device, then unwinding these fiber loops, and taking up the fiber with high speed has been proposed for fiber heat setting in U.S. Pat. No. 3,426,553 to Erb (1969), U.S. Pat. No. 3,774,384 to Richter (1973), U.S. Pat. No. 4,414,756 to Simpson et al. (1983), U.S. Pat. No. 5,522,161 to Vetter (1996), and U.S. Pat. No. 5,588,604 to Vetter et al. (1996). However, the invented method and apparatuses were not designed for and capable of fiber drawing.
U.S. Pat. No. 2,302,508 to Sordelli (1942) disclosed an apparatus substantially in the form of a winding frame having the general form of a frustum of a cone, which upon being set rotating about its axis promotes the winding of the filament material in a series of helical turns distributed over the apparatus from its end having the minimum diameter towards the opposite end having the maximum diameter. The filament material winds up in a continuous manner onto the apparatus and unwinds therefrom after it has traveled along the said series of turns; during the movement the material undergoes a continuous progressive stretching action, whereby it increases in length to an extent which depends upon the structural characteristics of the apparatus. The winding frame comprises a carrier member rotating about a central axis and a plurality of cantilever rollers each rotatably mounted at one end at said carrier member, the axes of said rollers being both diverged and skewed with respect to the central axis. The skewed rotated rollers draw the fiber by expanding the helical turns while conveying these turns along the central axis.
In this invention, the fiber passes the drawing apparatus at low speed providing longer drawing path and drawing time T as well as lower strain rate Vstrain in comparison with conventional industrial drawing processes. However, this apparatus has some disadvantages with respect to implementation on the industrial scale. They are as follows:
(1) It is complicated in design having the diverged and skewed cantilever conveyer-drawing members (driven rollers) rotated about their exes and simultaneously about the central axis as a part of the winding rotating frame.
(2) The apparatus has a fixed angle of divergency of the conveyer-drawing members and is not capable to change the fiber draw ratio, if it is necessary, by changing the angle of divergency.
(3) The conveyer-drawing members (which are cantilever, diverged, and skewed with respect to the central axis) are not strong enough to sustain high drawing forces in the drawing process while drawing large number of the fiber loops (up to a few hundreds) especially in case of high draw ratios (e.g., 5× and higher), high denier filaments, and present-day high tenacity fibers. Large number of loops (100-200 and higher) is necessary to provide long drawing time T and low strain rate Vstrain at high outlet speed Voutlet and throughput [see equation (32) below ]. For the same reasons, Sordelli's apparatus has also limitations to be long to place large number of the loops. Sordelli's apparatus has also limitations to provide large angle of divergency of the conveyer-drawing members and large diameter (and circumference) of the leading fiber loop at the delivery ends necessary for high draw ratios (5× and higher) because of design of the driving mechanism (gear box) to drive the conveyer-drawing members and possibility of sliding down of the fiber loops along the conveyer-drawing members (see below).
(4) In Sordelli's apparatus, the conveyer-drawing members (rollers) have smooth surface covered with rubber or other materials to improve friction and to prevent sliding down of the fiber loops on the surface of the members. In that case, it would be difficult to find the coating that can operate inside a heat chamber at elevated temperatures necessary for effective hot drawing of the fibers. Without the coating, it is quite possible that the loops will slide down the conveyer-drawing members especially in case of (i) the higher divergency of the conveyer-drawing members necessary for higher draw ratios, (ii) some polymers with lower friction coefficient, and (iii) some finishes applied in the fiber making process.
(5) Sordelli's apparatus is not operator-friendly, i.e., it is difficult to load the fiber end into the apparatus to start the drawing process as well as to restart the apparatus after fiber breakage, and, in case of fiber breakage, the broken ends can be easily wound on the rotated conveyer-drawing members (rollers) resulting in significant operational problem.
Thus, the Sordelli's apparatus has substantial disadvantages to be industrially feasible.